Solar cell comprising an oxide-nanoparticle buffer layer and method of fabrication

ABSTRACT

A buffer layer for protecting an organic layer during high-energy deposition of an electrically conductive layer is disclosed. Buffer layers in accordance with the present invention are particularly well suited for use in perovskite-based single-junction solar cells and double-junction solar cell structures that include at least one perovskite-based absorbing layer. In some embodiments, the buffer layer comprises a layer of oxide-based nanoparticles that is formed using solution-state processing, in which a solution comprising the nanoparticles and a volatile solvent is spin coated onto a structure that includes the organic layer. The solvent is subsequently removed in a low-temperature process that does not degrade the organic layer.

STATEMENT OF RELATED CASES

This case claims priority of U.S. Provisional Patent Application Ser.No. 62/245,260 filed on Oct. 22, 2015 and U.S. Provisional PatentApplication Ser. No. 62/398,220 filed on Sep. 22, 2016, each of which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under contractDE-EE0004946 awarded by the Department of Energy. The Government hascertain rights in the invention.

FIELD OF THE INVENTION

The present invention pertains generally to optoelectronics and, moreparticularly, to the formation of transparent electrodes for solarcells.

BACKGROUND OF THE INVENTION

A solar cell is an optoelectronic semiconductor device that converts theenergy of light incident upon it directly into electricity. The incidentlight is absorbed in an absorption layer of the solar-cell, which givesrise to the generation of free electrical carriers (i.e., electrons andholes). The free electrical carriers produce a voltage across theterminals of the device, which can be used to directly power electricalsystems or be stored in an electrical storage system (e.g., a battery,etc.) for later use.

In order to generate a free-carrier pair in the absorption material, aphoton must have energy greater than energy bandgap (EG) of thematerial. The EG of a material is the energy difference between the topof its valence band (the highest energy level populated by boundelectrons) and the bottom of its conduction band (the lowest energylevel populated by free electrons). When a photon is absorbed, itsenergy is given over to a bound electron-hole pair, which frees theelectron and enables it to go from the valence band into the conductionband. The energy of a photon is inversely proportional to its wavelength(E_(p)=hc/λ, where E_(p) is photon energy, h is Planck's constant, c isthe speed of light, and λ is wavelength); therefore, longer-wavelengthlight (e.g., red light) has lower photon energy than shorter-wavelengthlight (e.g., blue light). As a result, the choice of semiconductor usedto absorb the light has significant impact on the efficiency of a solarcell.

Silicon is perhaps the most commonly used absorbing material insolar-cells at present, due to its relatively low EG, highly developedfabrication infrastructure, and low cost as compared to othersemiconductor materials. Unfortunately, silicon does not efficientlyabsorb light. In addition, since the free electrons and free holes tendto occupy energy levels at the bottom of the conduction band and the topof the valence band, respectively, any extra energy that theelectron-hole pairs receive from higher-energy photons is converted intoheat that is transferred into the semiconductor material in a processreferred to as “thermalization.” Thermalization reduces theenergy-conversion efficiency of the solar cell and also raises thetemperature of the device, which can lead to degradation and lifetimeissues.

In the quest for improved device performance, the solar-cell communityhas been aggressively searching for material alternatives to silicon,and perovskites are now seen as being among the most attractive siliconsubstitutes. In fact, in recent years, perovskite-based solar cellefficiencies have become extremely competitive with silicon-baseddevices. Their rapid rise is the result of a unique combination ofproperties, including strong optical absorption and long ambipolardiffusion lengths enabled by the benign nature of their intrinsicdefects. In addition, perovskites are well suited for use in the topcell of a tandem solar-cell configuration, which enables improvedenergy-conversion efficiency and thermalization loss than can beachieved in more conventional single-cell devices.

A tandem solar cell is a stacked structure comprising a top photovoltaicportion that is made of a material having a relatively higher EG and abottom photovoltaic portion that is made of a material having arelatively lower EG. In other words, a tandem solar cell has two p-njunctions and two different band gaps. When light is incident on thestacked structure, high-energy photons are first absorbed in the topportion, while lower-energy photons pass through the top portion to beabsorbed in a bottom photovoltaic portion. This enables a broaderspectrum of light to be absorbed, thereby improving energy-conversionefficiency beyond the single-junction efficiency limit. In addition,thermalization in the bottom cell is reduced because of the absorptionof high-energy photons in the top cell. Depending on the EG of thematerial of the top solar cell, the fundamental efficiency limit forsilicon-based tandem solar cells can be as high as approximately39%—significantly higher than the theoretical efficiency limit of 33.7%for a single-junction silicon solar cell.

Perovskites are particularly attractive for use in the top cell intandem solar cell configurations having bottom cells comprising a widevariety of lower-EG materials (e.g., silicon, copper indium galliumselenide (CIGS), etc.) because perovskites have wide, tunable bandgapsand solution processability. As a result, perovskites offer a pathway toachieving industry goals of improving efficiency while continuing todrive down module cost.

Perovskite-based tandem solar cells have been demonstrated in bothmechanically stacked, four-terminal configurations and monolithicallyintegrated three-terminal configurations. Mechanically stacked tandemstructures have seen the largest success, recently reaching a powerconversion efficiency over 24%. The mechanically stacked architecturehas some advantages over monolithically integrated structures—mostnotably, it simplifies device fabrication, allows for silicon surfacetexturing, and requires no current matching. Monolithically integratedtandem structures, however, have greater promise due to the fact thatthey have fewer transparent electrode layers.

To date, however, the commercial viability of both single-junction andtandem perovskite-based solar cells has been limited due to thermal andenvironmental instability issues. Perovskites are susceptible tomoisture and methylammonium egress. In addition, halides in theperovskite material can react with metal electrodes, leading toelectrode corrosion. Efforts have been made in the prior art tostabilize the perovskite, such as using a hydrophobic heterojunctioncontact or providing an encapsulation layer that mitigates moistureingress or a pinhole-free metal oxide layer to prevent metal-halideinteraction. Little progress has been made for preventing methylammoniumegress, however.

In addition, the top electrode of a solar cell must be highlytransparent, as well as highly conductive. Due to fabricationconstraints, however, light must first pass through a hole-transportlayer (e.g., Spiro-OMeTAD) in a standard architecture, or anelectron-acceptor layer (e.g., [6,6]-phenyl-C61-butyric acid methylester ([60]PCBM), [6,6]-phenyl-C61-butyric acid methyl ester ([61]PCBM),bathocuproine (BCP), etc.) in an inverted architecture, before enteringthe perovskite, which gives rise to significant parasitic losses.

The de facto industry standard for transparent contacts is a sputteredindium tin oxide (ITO), which is typically deposited as thin film viaRF-magnetron sputtering. Unfortunately, the temperatures duringdeposition and post-annealing may accelerate methylamine evolution,resulting in irreversible damage of the perovskite active layer. Inaddition, the high energy of the sputtered electrode-material particlescan easily damage the perovskite and carrier-extraction layers (i.e.,Spiro-OMeTAD, PCBM, bathocuproine (BCP), etc.) during the sputteringprocess, leading to degradation of device performance.

The addition of a buffer layer on top of theperovskite/carrier-extraction layer stack prior to contact formation isviewed as a potential approach for mitigating the problems associatedwith ITO deposition. Unfortunately, prior-art buffer layers have poorlong-term stability due to their chemical reactivity with perovskitecompositions. In addition, prior-art buffer layers have been plagued bylow efficiency, which degrades fill factor and open-circuit voltage.Further, many prior-art buffer layers require additional vacuumprocesses, such as evaporation, which undesirably complicates solar-cellfabrication. Still further, some prior-art buffer layers are notcompatible with many desirable device architectures. Molybdenum oxide(MoO_(x)), for example, cannot be used in an inverted solar-cellarchitecture.

The ability to readily form high-quality, highly transparent contactwindows on perovskite-based solar-cell structures remains, as yet,undemonstrated in the prior art.

SUMMARY OF THE INVENTION

The present invention enables the formation of a transparent conductingelectrode (TCO), such as an indium tin oxide (ITO) contact, inoptoelectronic devices/circuits in which it would otherwise beproblematic or impossible. In accordance with the present teachings,this is accomplished via a buffer layer comprising solution-processedoxide nanoparticles. Embodiments of the present invention areparticularly well suited for use in single-junction and dual-junctionsolar cell architectures.

An illustrative embodiment of the present invention is aninverted-structure, single-junction, perovskite-based solar cellcomprising a transparent conducting electrode of sputter-deposited ITO,which is disposed on a buffer layer comprising a plurality ofwide-bandgap oxide-based nanoparticles. In the illustrative embodiment,the nanoparticles are zinc oxide nanoparticles; however, in someembodiments, the nanoparticles comprise a different oxide, includingmetal oxides and semiconductor oxides. In some embodiments, the oxidenanoparticles are doped with another material, such as aluminum,hydrogen, indium, gallium, etc., to improve their conductivity.

The inclusion of the nanoparticle buffer layer enables the ITO layer tobe deposited and annealed without resulting in damage to the perovskitelayer. In some embodiments, such as tandem solar cells, the ability toanneal the ITO material is particularly important because it improvesits crystallinity, which increases its electrical conductivity andoptical transparency.

The nanoparticle buffer layer is formed, using solution-based processesthat are performed at low temperatures (e.g., <200° C., and preferablywithin the range of approximately 60° C. to 75° C.), on a layerstructure that includes a perovskite-based absorption layer locatedbetween an n-type heterojunction layer that acts as anelectron-selective (i.e., hole blocking) layer and a p-typeheterojunction layer that acts as a hole-selective layer. The ability toform the buffer layer via low-temperature, solution-based processingsimplifies the overall fabrication process and mitigates degradation ofunderlying layers. This is particularly important in embodiments inwhich the buffer layer is formed on underlying organic materials, suchas in the illustrative embodiment.

In some embodiments, the perovskite absorption layer is included as thetop cell of a tandem solar cell. In some embodiments, the tandem solarcell includes a bottom cell comprising silicon. In some embodiments, thetandem solar cell includes a material other than silicon, such as CIGS,a lower EG perovskite, and the like. In some embodiments, a buffer layerin accordance with the present invention is incorporated in anotherdevice structure, such as an organic light-emitting diode (OLED),organic thin-film transistor (OTFT), an electrochromic, and the like.

An embodiment of the present invention is an optoelectronic devicecomprising: a first layer comprising an organic material; a second layercomprising a plurality of oxide nanoparticles, the oxide nanoparticlesof the plurality thereof being characterized by a wide bandgap; and afirst transparent conductive electrode; wherein the second layer isbetween the first layer and the first transparent conductive electrode.

Another embodiment of the present invention is an optoelectronic devicecomprising: a first layer comprising a first material, the firstmaterial comprising a first perovskite and having a first energy bandgap(EG); a first electrical contact that is in electrical communicationwith the first layer, the electrical contact being substantiallytransparent for a first light signal characterized by a first wavelengthrange; and a buffer layer that is between the electrical contact and thefirst layer, the buffer layer comprising plurality of oxidenanoparticles.

Yet another embodiment of the present invention is a method for formingan optoelectronic device, the method comprising: providing a firstorganic layer; forming a first buffer layer comprising a first pluralityof oxide nanoparticles; and forming a first transparent conductiveelectrode such that it is disposed on the first buffer layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a single-junction, perovskite-based solar cell inaccordance with an illustrative embodiment of the present invention.

FIG. 2 depicts operations of a method suitable for forming solar cell100.

FIG. 3A depicts plots of current density as a function of appliedvoltage for two solar cells having structures analogous to that of solarcell 100.

FIG. 3B depicts plots of efficiency over time for solar cells 302 and304.

FIG. 4 depicts a schematic drawing of a mechanically stacked tandemsolar cell in accordance with the present invention.

FIG. 5A depicts plots of the external quantum efficiency (EQE) fortandem solar cell 400.

FIG. 5B depicts J-V curves for solar cell 100 and filtered andunfiltered solar cell 402.

FIG. 6 depicts a schematic drawing of a cross-sectional view of thesalient features of another tandem solar cell in accordance with thepresent invention.

FIG. 7 depicts a schematic drawing of a cross-sectional view of thesalient features of another tandem solar cell in accordance with thepresent invention.

FIG. 8 depicts a schematic drawing of a cross-sectional view of anexample of a three-junction solar cell device in accordance with thepresent invention.

DETAILED DESCRIPTION

FIG. 1 depicts a single-junction, perovskite-based solar cell inaccordance with an illustrative embodiment of the present invention.Solar cell 100 includes substrate 102, bottom contact 104,hole-selective layer 106, absorption layer 108, electron-selective layer110 (also referred to as electron-acceptor layer 110), buffer layer 112,top contact 114, and antireflection coating 116, arranged as shown.Solar cell 100 generates output voltage, V1, when illuminated with lightsignal 118. Although the illustrative embodiment is a single-junctionsolar cell that incorporates a buffer layer in accordance with thepresent invention, it will be clear to one skilled in the art, afterreading this Specification, that the present invention is more broadlyapplicable to other solar-cell architectures (tandem solar-cellstructures, etc.), as well as for the fabrication of other structures,such as organic light-emitting diodes (OLEDs), organic thin-filmtransistors (OTFTs), and electrochromics, among others optoelectronicdevices.

FIG. 2 depicts operations of a method suitable for forming solar cell100. Method 200 begins with operation 201, wherein bottom contact 104 isformed such that it is disposed on conventional glass substrate 102. Forthe purposes of this Specification, the term “disposed on” (or “formedon”) is defined as “exists on” an underlying material or layer. Thislayer may comprise intermediate layers, such as transitional layers,necessary to ensure a suitable surface. For example, if a material isdescribed to be “disposed (or grown) on a substrate,” this can mean thateither (1) the material is in intimate contact with the substrate; or(2) the material is in contact with one or more transitional layers thatreside on the substrate.

Bottom contact 104 is a layer of ITO having a thickness suitable forproviding low electrical sheet resistance. A typical value for thethickness of bottom contact 104 is approximately 170 nanometers (nm);however, one skilled in the art will recognize that any practicalthickness can be used.

At operation 202, hole-selective layer 106 is formed on bottom contact104 to define a hole-selective contact for solar cell 100.Hole-selective layer 106 is a substantially smooth, hydrophilic layer ofpoly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS),which is formed by spin coating aqueous PEDOT:PSS onto bottom contact104 and curing it. As will be appreciated by those skilled in the art, avariety of other materials can be used to form this layer, including,without limitation, small molecules such as spiro-OMeTAD, otherpolymers, such as poly triarylamine (PTAA), and inorganic materials,such as NiO.

At operation 203, absorption layer 108 is formed on hole-selective layer104. In the depicted example, absorption layer 108 comprises aCH₃NH₃PbI₃ perovskite that is formed by dissolving lead(ii) acetate(PbAc) and methylammonium iodide (MAI) (1:3 molar ratio) inn,n-dimethylformamide (DMF) and spinning the resultant solution onto asubstrate. The layer is initially dried at room temperature and thenannealed at 100° C. for 5 minutes. One skilled in the art willrecognize, after reading this Specification, that myriad perovskitelayers can be used in absorption layer 108 without departing from thescope of the present invention. Other perovskites suitable for use inthe present invention include, without limitation,Cs_(x)FA_(1-x)Pb(Br_(y)I_(1-y))₃, pure formamidinium based perovskite(FAPbI₃), tin containing perovskitesCs_(x)FA_(1-x)Pb_(z)Sn_(1-z)(Br_(y)I_(1-y))₃ etc. Further, one skilledin the art will recognize that many alternative methods can be used toform a layer of perovskite without departing from the scope of presentinvention.

Absorption layer 108 and hole-selective layer 104 collectively define ap-type heterojunction within solar cell 100.

At operation 204, electron-acceptor layer 110 is formed by spin-coatingliquid-phase organic material onto the perovskite of absorption layer108 and curing it via a low-temperature anneal. In the depicted example,electron-acceptor layer 110 is a layer of [60]PCBM having a thickness ofapproximately 20 nm. It is preferable that the layer of PCBM be thin toensure good electron-extraction properties, while still achieving highoptical transmission. Electron-acceptor layer 110 and absorption layer108 collectively define an n-type heterojunction in solar cell 100. Insome embodiments, a layer of lithium fluoride (LiF) is included as apassivation layer between electron-acceptor layer 110 and absorptionlayer 108. Electron-acceptor layer 110 is formed. In some embodiments,PCBM also prevents the development of an extraction barrier. In someother embodiments, the organic material can be an electrically-activematerial, such as [70]PCBM, spiro-OMeTAD or a polymer organic, such asPTAA.

In some embodiments, electron-acceptor layer 110 includes a thin layerof bathocuproine (BCP), which is evaporated on the PCBM layer to improvehole blocking and electron extraction.

For convenience, the layers of solar cell 100 between bottom contact 104and buffer layer 112 (i.e., hole-selective layer 106, absorption layer108, and electron-selective layer 110, as well as any additionalassociated layers) are referred to herein, collectively, as perovskitecell 122.

At operation 205, buffer layer 112 is formed on electron acceptor layer110. Buffer layer 112 comprises a layer of oxide nanoparticles 120,which is formed by dispersing a solution comprising the nanoparticlesand isopropyl alcohol (IPA), spin coating the mixture onto layer 110 toform a nascent buffer layer, and enabling the removal of the solventfrom the solution (e.g., via evaporation at room temperature or at aslightly elevated temperature in an oven or on a hotplate).

Each of nanoparticles 120 comprises a wide-bandgap oxide, wherein “widebandgap” is defined as an effective energy bandgap greater than 2.0 eV.In the depicted example, nanoparticles 120 comprise zinc oxide that isdoped with approximately 2 mol % aluminum (i.e., AZO). Zinc oxide is aparticularly attractive host material of nanoparticles 120 due to itsdeep valence level of approximately −7.6 eV. In addition, the use ofaluminum-doped zinc oxide (AZO) nanoparticles reduces or eliminates thedevelopment of an extraction barrier that can arise from a misalignmentof the work functions of ZnO and the ITO of top contact 114, therebyachieving a more ohmic top contact. However, one skilled in the art willrecognize, after reading this Specification, that many alternativematerials can be used in nanoparticles 120 without departing from thescope of the present invention. Alternative materials suitable for usein nanoparticles 120 include, without limitation, tin oxide (SnO₂),titanium oxide (TiO₂), tungsten oxide (W₂O₃), aluminum oxide (Al₂O₃),silicon dioxide (SiO₂), nickel oxide (NiO), molybdenum oxide (MoO_(x)),etc. Further, those skilled in the art will understand that thenanoparticles can be doped with many materials other than aluminum;including, without limitation, hydrogen, indium, gallium, and the like.A characteristic of a suitable dopant is that it can be alloyed into thehost material (e.g., zinc oxide, etc.) and improve the conductivity ofthe host material. In some other embodiments, wide-bandgap oxidematerials are undoped.

It is an aspect of the present invention that forming buffer layer 112using solution-based processing avoids some of the complications thatarise in the prior art. For example, it is well known that bonds withina perovskite can be broken when material is deposited on the perovskitematerial using a high-energy process, such as sputtering. Sincesolution-based processing low-energy, its use enables buffer layer 112to be formed on the perovskite material of absorption layer 110 withlittle or no damage.

At operation 206, top contact 114 is formed on buffer layer 112 suchthat it is substantially transparent for light signal 118. For thepurposes of this Specification, including the appended claims,“transparent” is defined as having a transmittance equal to or greaterthan 60%, where transmittance is typically measured at a wavelength of550 nm, corresponding to the maximum of the human eye luminosity curve.To form top contact 114, a layer of ITO having a thickness within therange of approximately 60 nm to approximately 500 nm (preferablyapproximately 100 nm) is sputtered onto buffer layer 112 and thenannealed at a temperature less than or equal to 200° C. It is anotheraspect of the present invention that the presence and robustness of thelayers of buffer layer 112 enable the sputter deposition of the ITO as atransparent electrode with little or no damage to underlyingelectron-selective layer 110 or the perovskite material of absorptionlayer 108.

In some embodiments, another material suitable for forming asubstantially optically transparent, electrically conductive layer(e.g., AZO, IZO, HZO, etc.) is sputtered onto buffer layer 112.

At optional operation 207, antireflection coating 116 is formed on topcontact 114 via conventional evaporation. In the depicted example,antireflection coating 116 is a layer of magnesium fluoride (MgF₂)having a thickness of approximately 150 nm; however, any suitableantireflection coating can be used in embodiments of the presentinvention. In some embodiments, an additional antireflection coating isformed on the bottom surface of substrate 102.

Fully fabricated solar cells analogous to solar cell 100 were found tohave an open circuit voltage (Voc) greater than 0.9 V, which evinces theefficacy of buffer layer 112 for protecting the integrity of the organiclayers of the solar cell structure (i.e., absorption layer 108 andelectron-selective layer 110).

It should be noted that the materials and layer thicknesses providedherein are merely exemplary and that alternative materials and/or layerthicknesses can be used without departing from the scope of the presentinvention.

FIG. 3A depicts plots of current density as a function of appliedvoltage for two solar cells having structures analogous to that of solarcell 100. Plot 300 shows measured current density vs. applied voltagefor solar cells 302 and 304. Solar cell 302 includes a semi-transparentbottom contact 104 made of a substantially transparent layer of ITO.Solar cell 304 includes an opaque bottom contact 104 that includeslayers of aluminum and silver. Measurement data was taken whileilluminating solar cells 302 and 304 through aperture masks havingapertures of 0.39 cm² and 0.12 cm², respectively.

FIG. 3B depicts plots of efficiency over time for solar cells 302 and304.

The results shown in plots 300 and 306 demonstrate the efficacy ofbuffer layer 112 as a support layer for the formation of an operativetop contact using conventional sputtering without damage to theunderlying organic layers. First, the fill factor and voltage for solarcells 302 and 304 are substantially comparable, demonstrating that a topcontact 114 disposed on an oxide-nanoparticle-based buffer layer (i.e.,buffer layer 112) operates in normal fashion. Second, the fill factorand voltage for solar cells 302 and 304 are comparable. It should benoted that the lower current density measured for solar cell 302 can beattributed to the fact that its bottom contact does not function as aback reflector. Third, plot 306 shows that the inclusion of buffer layer112 enables operation of the solar cells at room temperature with astabilized power efficiency of 12.3% and 13.5% for solar cells 302 and304, respectively.

Table 1 below summarizes measurement data obtained for solar cells 302and 304.

TABLE 1 Photovoltaic parameters of single-junction perovskite-basedsolar cells having semi-transparent and opaque bottom contacts. J_(SC)[mA/cm²] VOC [mV] FF Efficiency [%] Solar Cell 302 16.5 952 0.77 12.3Solar Cell 304 18.8 938 0.77 13.5

As discussed above, in some embodiments, the host material ofnanoparticles 120 is doped to facilitate ohmic-contact operation of topcontact 114. By employing doped oxide-based nanoparticles in bufferlayer 112 (e.g., AZO), the work functions of the host material (e.g.,ZnO) and the material of top contact 114 (e.g., ITO) can be more closelyaligned. This mitigates the development of an extraction barrier thatcan impair operation of the solar cell.

It is an aspect of the present invention that the ability to form ahigh-quality transparent contact on a structure comprising organicmaterials enables device structures that were difficult, if notimpossible, to produce in the prior art. Examples of such structuresinclude, without limitation, mechanically stacked tandem solar cellsincluding a perovskite-based top cell, monolithically integrated tandemsolar cells including a perovskite-based top cell, monolithicallyintegrated tandem solar cells including perovskite-based top and bottomcells, and the like.

FIG. 4 depicts a schematic drawing of a mechanically stacked tandemsolar cell in accordance with the present invention. Solar cell 400includes perovskite solar cell 100, silicon solar cell 402, and adhesivelayer 404, which mechanically affixes the two solar cells in hybridfashion.

Silicon solar cell 402 is a conventional silicon-based solar cellcomprising bottom contact 406, silicon cell 408, and top contact 410.One skilled in the art will recognize that, like solar cell 100, aconventional silicon-based solar cell normally includes additionallayers, such as p-type and n-type amorphous silicon heterojunctionlayers, etc. In similar fashion to perovskite cell 122 described above,for convenience, the layers of silicon solar cell 402 between bottomcontact 406 and top contact 410 (i.e., absorption layer, heterojunctionand carrier-selective layers, etc.) are referred to herein,collectively, as silicon cell 408.

Solar cell 602 is fabricated in substantially conventional fashion,beginning with the texturing of the top surface of a silicon substrate,which functions as the absorption layer of silicon cell 408.

For example, silicon cell 408 begins as a silicon substrate that is anN-type, 280-μm-thick, double-side polished float-zone (FZ) wafer. Thetop surface of the substrate is textured by first depositing a250-nm-thick, low-refractive-index silicon nitride layer byplasma-enhanced chemical-vapor deposition (PECVD) on its bottom surfaceand exposing the unprotected top surface to potassium hydroxide (KOH).

After removal of the nitride layer from the bottom surface, intrinsicand n-type amorphous silicon (a-Si:H) layers (not shown) are depositedon the top surface of the substrate.

In similar fashion, intrinsic and p-type a-Si:H films layers (not shown)are deposited on bottom top surface of the substrate to complete siliconcell 408.

Bottom contact 406 is then formed by depositing a layer of silver (orother suitable electrically conductive material) having an appropriatethickness.

Top contact 410 is formed by depositing, in conventional fashion, alayer of a transparent electrically conductive material. In the depictedexample, top contact 410 comprises a layer of ITO having a thickness ofapproximately 500 nm.

Once silicon solar cell 402 is complete, it is joined with solar cell100 in conventional fashion. In the depicted example, the solar cellsare affixed via adhesive layer 404, which is a layer of electricallyinsulating epoxy that is substantially transparent for light signal 118.In some embodiments, solar cells 100 and 402 are mechanically joined viaanother conventional method.

FIG. 5A depicts plots of the external quantum efficiency (EQE) fortandem solar cell 400. Plot 500 shows measured EQE of individual solarcells 100 and 402 in response to light signal 118. Traces 502 and 504show the EQE of the individual silicon and perovskite solar cells,respectively, under direct illumination by the light signal such thatthe light incident on the solar cells spans the full range of 300 nm to1200 nm. Trace 506 shows EQE for individual solar cell 402 whenilluminated by light signal 118 after it has passed throughperovskite-based solar cell 100. Solar cell 402 is referred to as“unfiltered” when directly illuminated with light signal 118 and“filtered” when it is illuminated by the light signal after it haspassed through solar cell 100.

FIG. 5B depicts J-V curves for solar cell 100 and filtered andunfiltered solar cell 402.

Table 2 below summarizes measurement data obtained for solar cells 100and 402.

TABLE 2 Photovoltaic parameters of individual solar cells 100 and 402,as well as stacked tandem configuration 400. J_(SC) [mA/cm²] VOC [mV] FFEfficiency [%] Solar Cell 100 16.5 952 0.774 12.3 (individually) SolarCell 402 38.3 587 0.754 17.0 (unfiltered) Solar Cell 402 13.3 562 0.7625.7 (filtered) Tandem solar 18.0 cell 400

The data shows that the efficiency of unfiltered solar cell 402 is17.0%. It should be noted that the V_(OC) of solar cell 402 is limiteddue to excess shaded area from the aperture mask used during itsillumination, as well as the absorption of the light signal in topcontact 114. In the mechanically stacked configuration, in which lightsignal 118 passes through solar cell 100 prior to impinging on solarcell 402, the combined efficiency of the two solar cells is 18.0% with aJSC of 13.3 mA/cm² from the bottom cell (i.e., solar cell 402).

It is an additional aspect of the present invention that ananoparticle-based buffer layer, such as buffer layer 112, enables asputtered ITO contact layer that acts as a barrier for moisture ingress,methylammonium egress, while the buffer layer itself protects thecontact layer from halide-based corrosion.

It is yet another aspect of the present invention that the inclusion ofa buffer layer comprising oxide-based nanoparticles enablesmonolithically integrated tandem solar cells comprising perovskite-basedtop cells combined with bottom cells that can include any of a widerange of materials, such as silicon, CIGS, a lower EG perovskite, andthe like.

FIG. 6 depicts a schematic drawing of a cross-sectional view of thesalient features of another tandem solar cell in accordance with thepresent invention. Solar cell 600 includes solar cell 100 and siliconsolar cell 602, which collectively form a monolithically integratedtandem solar cell structure. For the purposes of this Specification,including the appended claims, the term “monolithically integrated” isdefined as formed by depositing layers on a single substrate via one ormore thin-film deposition processes and, optionally, patterning thedeposited layers after deposition. The term monolithically integratedexplicitly excludes structures wherein two or more fully fabricateddevices are joined, after their fabrication on separate substrates, toform a unitary structure.

Solar cell 602 is a silicon heterojunction solar cell that is analogousto solar cell 402 described above and with respect to FIG. 4; however,rather than conventional top contact 410, solar cell 602 includes centercontact 604, which is dimensioned and arranged to support the monolithicintegration of the layers of the solar cell 100.

The fabrication of solar cell 602, up to the formation of center contact604, is as described above and with respect to silicon solar cell 402.

To form center contact 604, a thin (e.g., 20-nm thick) layer of ITO isdeposited through a shadow mask to define multiple conductive regions(e.g., 1 cm by 1 cm). These ITO regions provide a recombination junctionbetween the silicon and perovskite cells with minimal parasiticabsorption. It should be noted that the ITO layer can be very thinbecause lateral conductivity is not of concern at this junction. Itshould also be noted that the shape of the ITO regions can be other thansquare.

Once an appropriate center contact has been formed, the thin-film layerstructure of solar cell 100 is formed on solar cell 602 in substantiallythe same manner as described above and with respect to FIGS. 1 and 2. Itshould be noted that, when formed in a monolithically integrated tandemconfiguration, substrate 102 is not included in solar cell 100.

It should be noted, however, that the processes used to fabricate theconstituent layers of solar cell 100 must be compatible with the solarstructure on which they are formed. For example, in some embodiments,hole-selective layer 106 comprises a material other than PEDOT:PSS, suchas NiO. An NiO layer, however, is typically annealed at 300° C. forseveral minutes after its deposition. Unfortunately, such an annealingstep would lead to hydrogen loss in the doped amorphous silicon layersand/or crystallization of the amorphous silicon layers of silicon solarcell 602 and compromise their passivation properties. As a result, inaccordance with the present invention, a hole-selective layer comprisingNiO is annealed at a lower temperature for a longer period of time(e.g., at 200° C. for 10 hours) than would normally be used in the priorart.

It is yet another aspect of the present invention that the formation ofan effective buffer layer and top contact using low-temperature,solution-based processing enables monolithically integratedmulti-junction solar cell structures (i.e., having two or morejunctions) that are not possible in the prior art—specifically,multi-junction configurations that include perovskite absorption layersin every one of the stacked cells.

FIG. 7 depicts a schematic drawing of a cross-sectional view of thesalient features of another tandem solar cell in accordance with thepresent invention. Solar cell 700 includes solar cell 100-1 and solarcell 100-2, which collectively form a monolithically integrated,completely perovskite-based tandem solar cell structure.

Each of solar cells 100-1 and 100-2 is analogous to solar cell 100,described above; however, perovskite cells 122-1 and 122-2 are formedsuch that the EG of perovskite cell 122-1 is lower than the EG ofperovskite cell 122-2. In some embodiments, at least one of perovskitecells 122-1 and 122-2 has a regular (i.e., not inverted) architecture.

In the depicted example, perovskite cell 122-1 comprises MAPbI₃perovskite having an EG of approximately 1.6 eV, while perovskite cell122-2 comprises MAPbBr₃ perovskite having an EG of approximately 2.3 eV.

Center contact 702 is analogous to top contact 114; however, in someembodiments, center contact 702 includes a plurality of separateconductive regions as described above and with respect to center contact604.

Solar cell 700 is formed using sequential applications of method 200,described above. It should be noted that, since all operations of method200 are performed at low temperature (i.e., ≤150° C.), there is notheoretical limit to the number of solar cell structures that can beincluded in a multi-junction solar cell. As a result, monolithicallyintegrated stacked solar cell devices having three or more junctions canbe made simply by employing method 200 the requisite number of times.

Table 3 below summarizes a portion of the design space for amulti-junction solar cell, such as solar cell 700.

TABLE 2 Theoretical performance for two-junction and three-junctionsolar cell structures. Si+ 1.48 eV Si+ Si+ Perov. + Si, 1.48 eV 1.60 eV2.0 eV 1.75 eV 2.0 eV 2.0 eV CIGS Perov. Perov. Perov. Perov. Perov.Perov. Dark Current (A) 4E−16 2.61E−21 4E−23 16.5 952 0.774 12.3Photocurrent (A) 0.044 0.0298 0.0254 0.0147 0.0211 0.0147 0.0143 Voc (V)0.83 1.12 1.23 1.569 2.18 2.69 3.52 SQ PCE (%) 32 30 28 27 41 36 45

FIG. 8 depicts a schematic drawing of a cross-sectional view of anexample of a three-junction solar cell device in accordance with thepresent invention. Solar cell 800 includes silicon solar cell 402 andsolar cells 102-1 and 102-2, which are monolithically integrated onsubstrate 406.

In order to fabricate solar cell 800, method 200 is performed twice. Inthe first performance of the method, a first perovskite absorption layeris formed such that it resides on layer stack 802 as part of perovskitecell 122-1. The first perovskite layer has an EG that is higher than theEG of silicon.

Method 200 is then repeated such that a second perovskite layer isformed such that it resides on layer stack 804 as part of perovskitecell 122-2. The second perovskite layer has an EG that is higher thanthe EG of the first perovskite layer.

As a result, each solar cell structure of solar cell 800 is operativefor converting the energy of a different portion of the wavelengthspectrum of light signal 118 into electrical energy.

It is to be understood that the disclosure teaches only examples of theillustrative embodiment and that many variations of the invention caneasily be devised by those skilled in the art after reading thisdisclosure and that the scope of the present invention is to bedetermined by the following claims.

What is claimed is:
 1. An optoelectronic device comprising: a substrate;a first contact disposed on and in physical contact with the substrate;a first layer comprising an organic material, the first layer beingdisposed on the first contact; a second layer that is configured toprotect the first layer during a sputter-deposition process, the secondlayer comprising a plurality of oxide nanoparticles, the oxidenanoparticles of the plurality thereof being characterized by a widebandgap, the second layer being disposed on the first layer such thatthe first layer is between the second layer and the first contact; and afirst transparent conductive electrode disposed on the second layer, thefirst transparent conductive electrode comprising a sputter-depositedmaterial; wherein the second layer is between the first layer and thefirst transparent conductive electrode.
 2. The optoelectronic device ofclaim 1 wherein the organic material is a perovskite material.
 3. Theoptoelectronic device of claim 1 wherein the oxide nanoparticles of theplurality thereof comprise an oxide selected from the group consistingof zinc oxide (ZnO), tin oxide (SnO₂), titanium oxide (TiO₂), tungstenoxide (W₂O₃), aluminum oxide (Al₂O₃), silicon dioxide (SiO₂), nickeloxide (NiO), and molybdenum oxide (MoO_(x)).
 4. The optoelectronicdevice of claim 3 wherein the oxide nanoparticles of the pluralitythereof are doped with a first dopant that is selected from the groupconsisting of aluminum, hydrogen, indium, and gallium.
 5. Theoptoelectronic device of claim 1 wherein the optoelectronic device isselected from the group consisting of a solar cell, a light emittingdiode, and an electrochromic.
 6. The optoelectronic device of claim 1wherein the first layer comprises a first perovskite having a firstenergy bandgap (EG), and wherein the optoelectronic device furthercomprises: a third layer comprising a first material having a second EGthat is lower than the first EG; and a second transparent conductiveelectrode that is between the first layer and the third layer, thesecond transparent conductive electrode being in electricalcommunication with each of the first layer and the third layer.
 7. Theoptoelectronic device of claim 6 wherein the optoelectronic device is amonolithically integrated tandem solar cell.
 8. The optoelectronicdevice of claim 7 wherein the first material comprises a secondperovskite.
 9. An optoelectronic device comprising: a first layercomprising a first material, the first material comprising a firstperovskite and having a first energy bandgap (EG), the first layer beingdisposed on a substrate; a first electrical contact disposed on thesubstrate, the first electrical contact being between the substrate andthe first layer; a second electrical contact that is disposed on and inelectrical communication with the first layer, the electrical contactbeing substantially transparent for a first light signal characterizedby a first wavelength range, and the second electrical contactcomprising a sputter-deposited material; a first carrier-transport layerthat is disposed on and in physical contact with the first layer; and abuffer layer that is configured to protect the first layer during asputter-deposition process, the buffer layer between the secondelectrical contact and the first carrier-transport layer, the bufferlayer comprising plurality of oxide nanoparticles.
 10. Theoptoelectronic device of claim 9 wherein the oxide nanoparticlescomprise a wide-bandgap oxide that is selected from the group consistingof tin oxide, zinc oxide, aluminum-doped zinc oxide (AZO), titaniumoxide, nickel oxide, platinum oxide, tungsten oxide, and vanadium oxide.11. The optoelectronic device of claim 9 wherein the firstcarrier-transport layer is a hole-selective layer, and wherein theoptoelectronic device further comprises: a first electron-selectivelayer, the first layer being between the first carrier-transport layerand the first electron-selective layer; and a second electrical contactthat is in electrical communication with the first layer; wherein thefirst layer, the first carrier-transport layer, the firstelectron-selective layer, and the first and second electrical contactsare collectively operative for generating electrical energy based onlight characterized by the first wavelength range.
 12. Theoptoelectronic device of claim 11 further comprising: a second layercomprising a second material having a second EG that is lower than thefirst EG; a second hole-selective layer; a second electron-selectivelayer, the second layer being between the second hole-selective layerand the second electron-selective layer; and a third electrical contactthat is in electrical communication with the second layer; wherein thesecond electrical contact substantially transparent for the firstsignal; wherein the first light signal includes light characterized by asecond wavelength range; wherein the second layer, the secondhole-selective layer, the second electron-selective layer, and thesecond and third electrical contacts are collectively operative forgenerating electrical energy based on light characterized by the secondwavelength range; and wherein the first layer, second layer, and secondelectrical contact are arranged such that the second layer receives atleast a portion of the first light signal through the first layer andthe second electrical contact.
 13. The optoelectronic device of claim 12wherein the first layer and the second layer are monolithicallyintegrated.
 14. The optoelectronic device of claim 12 wherein the secondmaterial is selected from the group consisting of silicon, copper indiumgallium selenide (CIGS), and a second perovskite.
 15. The optoelectronicdevice of claim 1 wherein the sputter-deposited material is selectedfrom the group consisting of indium tin oxide (ITO), aluminum-doped zincoxide (AZO), indium zinc oxide (IZO), and hafnium zinc oxide (HZO). 16.The optoelectronic device of claim 1 wherein the sputter-depositedmaterial includes indium tin oxide.
 17. The optoelectronic device ofclaim 1 wherein the first transparent conductive electrode is inphysical contact with the second layer.
 18. The optoelectronic device ofclaim 9 wherein the first carrier-transport layer is anelectron-selective layer, and wherein the optoelectronic device furthercomprises: a first hole-selective layer, the first layer being betweenthe first carrier-transport layer and the first electron-selectivelayer; and a second electrical contact that is in electricalcommunication with the first layer; wherein the first layer, the firstcarrier-transport layer, the first hole-selective layer, and the firstand second electrical contacts are collectively operative for generatingelectrical energy based on light characterized by the first wavelengthrange.
 19. The optoelectronic device of claim 9 wherein thesputter-deposited material is selected from the group consisting ofindium tin oxide (ITO), aluminum-doped zinc oxide (AZO), indium zincoxide (IZO), and hafnium zinc oxide (HZO).
 20. The optoelectronic deviceof claim 9 wherein the sputter-deposited material includes indium tinoxide.